<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/jq-1.12.4.js"></script>
    <script>
        $(function () {
            $("#app").hover(
                //mouseover
                function () {
                    $(this).css("background-color", "yellow")
                },
                //mouseout
                function () {
                    $(this).css("background-color", "blue")
                })
        })
    </script>
    <style>
        #app {
            width: 100px;
            height: 100px;
            background-color: blue;
        }
    </style>
</head>

<body>
    <div id="app"></div>
</body>

</html>